The Harken Forged Bow Shackles used on Harken® blocks
are available separately. The number on the shackle denotes the screw diameter
in millimeters.

The breaking strengths on the Harken Forged Bow Shackles are
derived from tests that support 80% of the length of the screw pin that is unsupported,
which is similar to the area of a post in a block. Safe working loads are no
more than half the minimum breaking strength.